Wastewater treatment has always been important, particularly in agricultural production and food processing, which produces wastewater containing high concentrations of organic matter.
Anaerobic digestion is one method of treating wastewater containing high concentrations of organic matter. Through anaerobic digestion, large quantities of organic matter are removed from the wastewater by converting the organic matter into biogas. Anaerobic digestion is particularly suitable for wastewater from agricultural production and processing.
Inducing a sludge blanket or sludge bed (the terms “bed” and “blanket” are used interchangeably throughout this disclosure) has long been recognized as a way to promote high-rate anaerobic digestion. A sludge bed is used to retain anaerobic microorganisms—the most numerous of which are bacteria in a designated space. One prior method of establishing such a sludge bed is to utilize a so-called up-flow anaerobic sludge blanket (UASB), which causes a sludge blanket to form over time inside a bioreactor. There are many problems, however, with respect to prior UASBs. One significant problem with conventional UASB bioreactors relates to plugging of the gas/solid/liquid separator that is part of every UASB. The gas/solid/liquid separator retains bacteria within the reactor vessel while allowing gas and liquid to pass through. Conventional gas/solid/liquid separators can be problematic particularly when treating wastewater containing significant amounts of solids, such as animal wastewater or food wastes.
Still others have developed bioreactors wherein relatively high concentrations of bacteria are maintained by adding fixed media, such as plastic rings or rocks, which provide locations to which the bacteria can attach. Unfortunately, these prior types of bioreactors also plug often when treating substrates like animal manure and various food processing wastes.
Anaerobic bioreactors can be an efficient and effective way to treat wastewater while generating usable biogas in the process. But these benefits of bioreactors depend on reliable operation combined with minimal maintenance and low cost of operation. Minimized maintenance is realized in large part by minimizing mechanical complexity and the tendency of gas/solid/liquid separators to plug. This is because such bioreactors are generally operated as sealed units; opening of such bioreactors for maintenance generally results in ceased operations and a time-consuming re-start process. Plugged bioreactors generally must be opened for maintenance. Various types of septums are used in bioreactors to, among other things, prevent or minimize such plugging. But effective septum designs have been elusive and largely ineffective, and have been found overly complex and thus costly in terms of maintenance and service.
Previous gas/liquid/solid separator devices have been used in high-rate anaerobic bioreactor. One of these has helped to reduce the plugging of bioreactors is a rigid or semi-rigid partition or septum and auger device that is positioned inside an enclosed bioreactor vessel, such as disclosed in U.S. Pat. No. 7,452,467. The septum generally slopes upwardly from the sides of the vessel toward a central aperture or hole so that biogas produced below the partition can move along the bottom of the septum and escape into the top of the tank where it can be removed. An auger mechanism is incorporated with the septum to force solids down below or pull solids above the septum to control the amounts of solids retained in the bioreactor vessel. A mixer may be attached to the shaft of the auger to mix the bioreactor contents and prevent a crust from forming at the top of the bioreactor. Unfortunately, experience has demonstrated that such septum/auger devices remain subject to plugging, are difficult to maintain and service, and unacceptably increase the mechanical complexity of bioreactors in which they are used. In particular, experience has demonstrated that the aperture of such a septum is subject to plugging either with or without an auger mechanism. These devices also increase the complexity of operation of the bioreactor and require a penetration through the roof of the bioreactor with a motor and gear box to facilitate a rotating auger.
There is a need, therefore, to provide a bioreactor with a septum that is resistant to plugging, that minimizes mechanical complexity, and that simplifies bioreactor maintenance. Yet another need exists to provide a bioreactor with a septum that assists in separating settling solids from the liquid or wastewater in which the settling solids are suspended. Another need exists for a septum that aids in recirculation of any solids that do not break down, and to prevent the effluent exiting the bioreactor from being plugged. Another need exists for a septum that aids in protecting against overpressure of produced biogas.